Is it the lower oxygen level or lower atmospheric pressure that causes a reduction in power at altitude? I live at ~5k ft and would really like to install pods and do boom tube tips on my little 620. With stock tuning, will it be as lean as it seems? Or could it be already running a little rich because of the lower pressure/lower o2? Do I have it backward?
Thanks
its a density thing. air at atitude begins to be less and less dense. less oxygen = less fuel = less power.
less air (same fuel) = richer mix
620's are fuel injected, and adjust the mixture based on air pressure (altitude).
Less power at higher altitude due to less oxygen, and lower cylinder pressure.
Just to answer your first question the oxygen "level" is not affected by altitude as fresh air will be about 20.8% oxygen regardless of where you are on the planet.
The lower atmospheric pressure results in a lower number of oxygen molecules in a given space (for example 620 cc's) or as He Man said "it's a density thing". You can't get as much HP out of your 620 but your tee shots are to die for.
Thanks guys.
I didn't realize the ECU adjusted the fuel mixture due to air pressure. It does this, but doesn't do it for air flow?
The ECU doesn't directly measure airflow.
It 'calculates' it from engine rpm, throttle opening, air pressure, and air temperature.
Boom tubes on a 620? Now that's something I'd like to see. How do they look with the DSS?
GOOD fuel injection would have a mass airflow sensor and the pressure temperature sensor behind the throttle body.
Ducati's solution utilizes a comparison between atmospheric pressure & temperature and the throttle body opening to estimate mass flow of air into the engine. I suppose this cuts down on the number of sensors needed because fuel injection systems typically utilize a TPS regardless. It's an inferior system that is primarily reactive (in closed loop modes using the O2 to adjust) and predicted (in open loop following a set formula) as opposed to a proactive system in all cases (using as MAS primarily an the O2 sensor in the exhaust for fine tuning).
But, back to the original question, lower air pressure at high altitudes means less dense air which means less O2 (mass) per given volume of air. More O2 means you can burn more fuel at the proper ratios. Burning more fuel means more power.
To put it in a feel perspective... in Indiana at about 600ft gassing the throttle on my s2r in first always results in a massive wheelie... In north west New Mexico at about 6000ft it takes some skill to wheelie... at 10-12k at and around Ouray Colorado, it's almost impossible to wheelie.
Quote from: RUFKM on January 26, 2010, 11:25:00 PM
Just to answer your first question the oxygen "level" is not affected by altitude as fresh air will be about 20.8% oxygen regardless of where you are on the planet.
This is not true. Mountain climbers carry oxygen tanks to combat hypoxia because the percentage of oxygen in the atmosphere at high altitudes is in fact lower.
But as others here have illustrated, the issue is a bit more complicated than that; carb vs efi, etc.
A naturally aspirated engine tuned correctly for sea level will run rich at high altitudes. That was one of the problems associated with the early manually adjustable fuel injection systems that came out years and years ago before electronic systems.
Quote from: iamhybris on January 27, 2010, 06:48:17 AM
Boom tubes on a 620? Now that's something I'd like to see. How do they look with the DSS?
It won't be boomtubes (unfortunately they aren't made by motocreations for the 620), but a similar concept with tips and baffles.
Quote from: NorDog on January 27, 2010, 08:06:42 AM
This is not true. Mountain climbers carry oxygen tanks to combat hypoxia because the percentage of oxygen in the atmosphere at high altitudes is in fact lower.
I think the percentage of oxygen in the air remains the same at different altitude. But the air density gets lower the higher you climb. Lower air density = proportionally less oxygen molecures per breathe.
Quote from: Goat_Herder on January 27, 2010, 08:28:20 AM
I think the percentage of oxygen in the air remains the same at different altitude. But the air density gets lower the higher you climb. Lower air density = proportionally less oxygen molecures per breathe.
Yeah, I think you're right.
[UPDATE] Let me rephrase: You ARE right; I'm a dunderhead. [bang]
Atmosphere content does change with altitude... but not in the realm of human existence. The shuttle operates above the level where the atmosphere is no longer well mixed.
Everything you want to know about the atmosphere:
http://en.wikipedia.org/wiki/Earth_atmosphere (http://en.wikipedia.org/wiki/Earth_atmosphere)
Thanks for all of the comments. I feel smarter already. ;D
If ECU makes adjustments for pressure and gets an indirect sense of airflow, does it just not adjust for airflow? Is that the reason it runs lean with less back pressure (aftermarket pipes) and increased airflow (cut or removed air box) and requires ECU adjustments, or PCIII, etc.?
Quote from: Fergus on January 27, 2010, 12:45:19 PM
Thanks for all of the comments. I feel smarter already. ;D
If ECU makes adjustments for pressure and gets an indirect sense of airflow, does it just not adjust for airflow? Is that the reason it runs lean with less back pressure (aftermarket pipes) and increased airflow (cut or removed air box) and requires ECU adjustments, or PCIII, etc.?
Bingo! Not to mention being tuned lean to begin with to meet emissions...
OK. I was kind of hoping that I was already running rich because of the altitude (same fuel, less o2) and I could modify the exhaust and air filter and not be too far off tune. But I was wrong :-\. I'll look at PCIII and/or ECU re-map.
Thanks again [drink]
Quote from: NorDog on January 27, 2010, 08:06:42 AM
This is not true. Mountain climbers carry oxygen tanks to combat hypoxia because the percentage of oxygen in the atmosphere at high altitudes is in fact lower.
=fail.
The percentage of oxygen remains the same at sea level as it is at the top of Mt. Everest at about 20.8%. The partial pressure of oxygen (the pressure exerted against lung tissue by the oxygen content alone) decreases below acceptable limits at extreme altitudes. By using oxygen tanks climbers (or aviators) are increasing the partial pressure of oxygen against the lung tissue by having a higher oxygen content at the same (ambient) pressure. The other alternative is to compress the existing air at altitudes to a pressure near that of sea level = pressurized cabin.
Please do the homework before you trash me again.
Quote from: RUFKM on January 27, 2010, 06:59:01 PM
=fail.
The percentage of oxygen remains the same at sea level as it is at the top of Mt. Everest at about 20.8%. The partial pressure of oxygen (the pressure exerted against lung tissue by the oxygen content alone) decreases below acceptable limits at extreme altitudes. By using oxygen tanks climbers (or aviators) are increasing the partial pressure of oxygen against the lung tissue by having a higher oxygen content at the same (ambient) pressure. The other alternative is to compress the existing air at altitudes to a pressure near that of sea level = pressurized cabin.
Please do the homework before you trash me again.
Yeah, I already gave my mea culpa on this one. Thanks for sharing though.
FTR, my mistake is worse than it appears. I have professional training and experience by which i should know better. Actually, I do know better. The phrase "Terminal Brain Fart" comes to mind.
[bang]
Great use of "fail" though.
Quote from: NorDog on January 27, 2010, 07:29:00 PM
Yeah, I already gave my mea culpa on this one. Thanks for sharing though.
No worries NorDog.
Back to the 620 - if it's fuel injected and has an O2 sensor can Fergus run the FatDuc O2 Manipulator to adjust for his local conditions? It's been great for many of us in adjusting the A/F ratio and at $86 delivered to my door it's by far the most cost-effective mod I've done.
Just for the record... but the %oxygen begins to decrease at an irrelevantly high altitude. Probably where the confusion came from. ;)
Quote from: some random guy from the internetsThe homosphere and heterosphere are defined by whether the atmospheric gases are well mixed. In the homosphere the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.[3] The homosphere includes the troposphere, stratosphere, and mesosphere. Above the turbopause at about 100 km (62 mi; 330,000 ft) (essentially corresponding to the mesopause), the composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.
yeah okay
i'll remember not to ride my make the beast with two backsin duc too muck over 330,000 feet for the risk of oxygen % decreasing
that's it i'm outa here or as i say RUFKM
i was going to type a lot to explain how it works, but just couldn't be bothered. here's one i prepared earlier - http://www.bikeboy.org/fuelinjection.html (http://www.bikeboy.org/fuelinjection.html)
wow...
looks like some one got sensitive last night.
Quote from: ducpainter on January 28, 2010, 03:10:05 AM
wow...
looks like some one got sensitive last night.
Well, speaking only for myself, or rather, of myself, it seems that the most important case of low oxygen percentage around here lately has been the one in my brain.